In less than five decades, summers across most of the globe could be hotter than any summer experienced by people to date, researchers have estimated.
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If climate change continues on its current trajectory, the probability that summers between 2061 and 2080 will be warmer than the hottest on record stands at 80 percent across the world's land areas, excluding Antarctica, which was not studied, the researchers said.
If greenhouse gas emissions are reduced, however, that probability drops to 41 percent.
"Extremely hot summers always pose a challenge to society," said lead author of the study Flavio Lehner, scientist at the US National Center for Atmosperic Research (NCAR).
"They can increase the risk for health issues, and can also damage crops and deepen droughts. Such summers are a true test of our adaptability to rising temperatures," Lehner noted.
The research team used two existing sets of model simulations to investigate what future summers might look like.
They created both by running the NCAR-based Community Earth System Model 15 times, with one simulation assuming that greenhouse gas emissions remain unabated and the other assuming that society reduces emissions.
By using simulations created by running the same model multiple times, with only tiny differences in the initial starting conditions, the scientists could examine the range of expected summertime temperatures for future "business-as-usual" and reduced-emissions scenarios.
The results showed that between 2061 and 2080, summers in large parts of North and South America, central Europe, Asia, and Africa have a greater than 90 percent chance of being warmer than any summer in the historic record if emissions continue unabated.
That means virtually every summer would be as warm as the hottest to date.
The findings will be published in an upcoming issue of the journal Climatic Change.
In some regions, the likelihood of summers being warmer than any in the historical record remained less than 50 percent, but in those places -- including Alaska, the central US, Scandinavia, Siberia and continental Australia -- summer temperatures naturally vary greatly, making it more difficult to detect effects of climate change, the researchers said.
Reducing emissions would lower the global probability of future summers that are hotter than any in the past, but would not result in uniformly spread benefits.
In some regions, including the US East Coast and large parts of the tropics, the probability would remain above 90 percent, even if emissions were reduced, the findings showed.
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But reduced emissions would result in a sizable boon for other regions of the world.